A detector tube of the above-mentioned kind is disclosed in U.S. Pat. No. 4,259,287. In this detecting tube, a pretreatment region is provided for detecting sodium hydroxide and/or calcium oxide aerosols. This pretreatment region is impregnated with NH.sub.4 Cl. The impregnation forms NH.sub.3 from the aerosols which leads to a coloration in the adjacent indicating layer which is impregnated with an acid and bromophenol blue.
A further detector tube for detecting hydrogen includes a pretreatment region which is provided with a palladium catalyzer. The pretreatment region forms water vapor with the hydrogen to be detected and with the atmospheric oxygen. The water vapor leads to a coloration in the indicator layer impregnated with selenium dioxide and sulfuric acid. In this connection, reference may be made to the publication entitled "Detector Tube Handbook" of Dragerwerk AG, Lubeck, May 1985, page 157.
The formation of the products in the pretreatment region occurs quantitatively so that the yield on the one hand is determined by the stoichiometric composition of the pretreatment substance and, on the other hand, from the effectiveness of the catalyzer. If the quantity of the material to be detected exceeds the capacity of the pretreatment substance, the excess portion of the material to be detected passes into the indicating layer without contributing to the coloration there. The consequence is a false measuring result.
On the other hand, the catalyzer which is utilized can be contaminated by an excessive loading of the material to be detected so that its effectiveness diminishes or ceases altogether. As a consequence, the catalyzer allows the material to be detected to pass the pretreatment region unchanged so that a subsequent coloration does not occur in the indicating region. In this way too, the measuring result is falsified.